by resonant inelastic X-ray scattering*

Magnetic metals with kagome structure can host various topologically non-trivial spin or electronic states, providing an extraordinary platform for studying the fundamental physicsof quantum materials. The metallic kagome ferromagnet Fe₃Sn₂, which has large spin-orbital coupling, shows considerable interplay between magnetism and non-trivial electronic states. The material is a host of anomalous bulk properties, including a first-order spin reorientation transition(1), an anomalous planar Hall effect(2), and field tunable electronic states(3, 4). Using the MCD RIXS and XAS (X-ray absorption spectroscopy), we report a flat spin wave band with large (compared to elemental iron) orbital moments in Fe₃Sn₂(5). The flat mode energy is consistent with the high Curie temperature (  K) as well as the strong acoustic mode dispersion. Our results illustrate how the magnetic circular dichroism analysis enhances the sensitivity of RIXS to magnetic excitations in ferromagnetic materials and unveil an unconventional flat spin wave band in a kagome ferromagnet.